Hi and welcome to Crackberry!
To unlock a Blackberry you will need the unlock code for your Blackberry Curve 8520.
Each unit has its own unlock code and any unlocking service will need your IMEI and another information as the MEP ID or your model and carrier to get the unlock code for your device.
You can find many unlocking services and people who can get your Blackberry unlocked here at Crackberry.